Modern electronic document processing systems (copiers, scanners and printers) generally include input scanners for electronically capturing the general appearance (for instance, the human readable information content and the basic graphical layout) of human readable hardcopy documents; programmed computers for enabling users to create, edit and otherwise manipulate electronic documents; and printers for producing hardcopy, human readable renderings of electronic documents. These systems typically have convenient access to mass memory for the storage and retrieval of electronic document files. Moreover, they often are networked by local area networks (LANs), switched data links, and the like for facilitating the interchange of digital electronic documents and for providing multi-user access to shared system resources, such as high speed electronic printers and electronic file servers.
Prior art document processing systems present a particular challenge to the system designer, since it is essential to preserve the relative input values of the original throughout the many stages of processing required to form the desired digital image. In particular, the system electronics inherently includes analog components, which are especially susceptible to drift; frequent recalibration is thus essential.
The calibration process itself is frequently complex. Calibration is typically initially performed at the factory, where sophisticated instruments facilitate the process. Even then, because of the unique demands of modern electronic document processing systems, particularly color processing systems, the process is often time consuming, and thus costly. Further, it is generally necessary to repeat the calibration process throughout the life of the document processing system. Typically this is performed at least once at the start of a scanning run, and often is at least semi-automated. However, some systems lose calibration during the scan, and become increasingly unreliable as the run proceeds. Others are still less than satisfactory when applied to the demanding environment of color scanner calibration.
A major source of error arises from the electro-optic detector system itself. Such systems generally comprise a detector array for receiving light reflected from or transmitted through a document, and for converting that light into electric signals indicative of the intensity of the reflected or transmitted light. The intensity of the reflected or transmitted light that is measured is a function not only of the “true” intensity arising from the characteristics of the image itself, but also from extraneous interference or noise caused primarily by undesired variations in the illumination and in the characteristics of the detector array.
Additional error arises in the electronic processing circuitry that receives the output of the electro-optic detector and converts to a usable output signal, generally in digital form. Such circuitry is subject to electronic noise and drift.
Consequently, it would be a significant improvement if the error present in prior art electronic document processing systems could be minimized or eliminated by reducing the impact that illumination errors, interference, noise or drift has on the performance of the document processing system. It would also be a significant improvement if the hardcopy output of electronic document processing systems provided an unambiguous source of information that could be used to determine distortions in the print from nominal such as spatial distortions and deviations from nominal in image properties (for example, color and font) to enable the calibration and correction of document processing systems and consequent generations of copies of the document. Thus, there is a need to overcome these and other limitations of the prior art and to provide an efficient method for calibrating and correcting electronic document processing systems and document prints and copies. The present invention, as illustrated in the following description, is directed to solving one or more of the problems set forth above.